So this isn't a great answer, this is kind of a starting answer for somebody else to take on and refine this better. But I'll make a stab at it.
First I have a question: Are you trying to retain this in a view? I don't think you can for what you're wanting to do, it's kinda complicated, so let's examine the operations that you need to do to actually do what you want.
You stated that you want the first 12 rows to be static every time, and they should always have their last column set as NULL, and the others should retain their value. So that's a business rule that we need to encode in SQL. But before we encode this as a rule, let's ask if there's a way to ENSURE that those 12 rows are the RIGHT rows every time. If we can make that assumption, then we can do this as part of the next step.
You're next requirement is to do a calculation on each row with the previous row. Since the first 12 rows are static (and I presume not calculated) then we don't have to ask "what about the first row". So the easiest way to do calculations on the previous row is to assign a rownum to each row, then use the rownum ID in a comparison. This meshes with the previous requirement.
So we should start by doing our select and assigning a rownum as well, like this:
SELECT
ROW_NUMBER() OVER (ORDER BY in.I_Date) AS rownum,
in.I_Date ,--Date
in.I_O_P ,--Money
in.I_O_H ,--Money
in.I_O_L ,--Money
in.I_C_O ,--Money
c.AMPS12_C --Money
CAST(0.0 AS Money) AS C12WR
FROM
dbo.IC_Raw_In in
INNER JOIN
dbo.AMPS12_C c ON in.I_Serial = c.i_serial
But for the way I would do this, I would funnel these values into a temp table, and then use that to work out what I need. That way you can just refer to the columns in subsequent calls, like this:
UPDATE t
SET C12WR = NULL
FROM temptable t
WHERE t.rownum < 12 -- see how we set the values = null here?
UPDATE t
SET C12WR = 510.3958
FROM temptable t
WHERE t.rownum = 12 -- see how we set the value to something static?
-- If this were a stored procedure we could use a value passed in here
and then we continue with:
UPDATE t
SET C12WR = ( ( t2.C12WR * 11.0 ) + t.I_C_O ) / 12.0
FROM temptable t
INNER JOIN temptable t2 ON t.rownum = (t2.rownum - 1) -- this let's us get the previous row
WHERE t.rownum > 12
Using this logic: After the 13th row, the C12WR column = (prevrow.C12WR * 11 + currow.I_C_O Column) / 12
And then you would just return the values that you wanted from the temptable.
Notice: the things I left off. I did not define the temp table, I did not get rid of the temptable. I did not use appropriate syntax for the temptable addressing. I did not validate anything. I presumed that this was going to be used in a stored procedure. I did not illustrate how to use the static value as a stored procedure passed parameter.
Hope this helps. Hope someone else helps make this a better answer ;)
Best Answer
Use a window function